When a linear guide, which is a direct acting apparatus, is driven, then the exfoliation occurs on a track surface and/or a surface (sliding surface) of a rolling member, and the linear guide is exposed to the exfoliated matter or the like. If the linear guide is exposed to the foreign matter such as the exfoliated matter or the like brought about as described above, then the foreign matter enters the track surface and/or the sliding surface, and problems arise such that the vibration becomes conspicuous upon the direct acting and/or the service life of the linear guide is shortened. In view of the above, for example, Patent Literature 1 discloses a technique concerning a direct acting apparatus provided with a detecting device for detecting a metal foreign matter. In the case of this technique, a pair of electrodes are arranged at a portion at which the foreign matter is easily accumulated in the direct acting apparatus. When the foreign matter is progressively accumulated at the portion, then a short circuit is electrically formed between the electrodes on account of the conductivity of the foreign matter, and the presence of the foreign matter is consequently detected.
Further, if the foreign matter includes the magnetic material such as iron or the like, the foreign matter can be detected by utilizing the magnetic force (magnetic flux density) of a permanent magnet as well. For example, when the magnetic flux density, which is provided in a predetermined magnetic circuit, is detected by utilizing a Hall element, if the foreign matter, which includes the magnetic material, invades a part of the structure of the magnetic circuit, then the magnetic flux, which comes from the permanent magnet, arrives at the Hall element in series while passing through the foreign matter existing in the magnetic circuit. As a result, the magnetic flux density, which is detected by the Hall element, changes. Therefore, the invasion of the foreign matter into the magnetic circuit is detected on the basis of the change.